Method for providing strong tough metal alloys

Abstract

A method for improving the strength and toughness of an austenitic metal alloy selected from the group consisting of stainless steel alloys of the AISI 200 and 300 series and non-stainless steel alloys containing iron, manganese, chromium, and carbon, said alloy having an Md temperature of no higher than about 100° C and an Ms temperature of no higher than about minus 100° C comprising the following steps: A. deforming the alloy at a strain of at least about 10 percent at a temperature in the range of about Md minus 50° C to about Md plus 50° C, said Md temperature being that of the alloy undergoing deformation, in such a manner that the material has a martensite phase of no greater than about 10 percent by volume and an austenite phase of at least about 90 percent by volume; and B. deforming the material produced in step (a) at a strain of at least about 10 percent and at a temperature no higher than minus 75° C in such a manner that the material has a martensite phase of at least about 50 percent by volume and an austenite phase of at least about 10 percent by volume; and An aged crystalline microstructure consisting essentially of a stainless steel alloy of the AISI 300 series having a martensite phase of at least about 50 percent by volume and an austenite phase of at least about 10 percent by volume and wherein the tensile strength of the alloy is at least about 190,000 pounds per square inch where the microstructure contains 50 percent martensite by volume and the tensile strength increases by at least about 2000 pounds per square inch for each additional percent of martensite above 50 percent.

We claim: 
     
       1. A process for improving the strength-toughness characteristics of an austenitic metal alloy selected from the group consisting of stainless steel alloys of the AISI 200 and 300 series and non-stainless steel alloys containing iron, manganese chromium, and carbon, said alloy having an Md temperature of no higher than about 100° C and an Ms temperature of no higher than about minus 100° C comprising the following steps: a. deforming the alloy at a strain of at least about 10 percent and at a temperature in the range of about Md minus 50° C to about Md plus 50° C, said Md temperature being that of the alloy undergoing deformation, in such a manner that the material has a martensite phase of no greater than about 10 percent by volume and an austenite phase of at least about 90 percent by volume; and   b. deforming the material produced in step (a) at a strain of at least about 10 percent and at a temperature no higher than about minus 75° C in such a manner that the material has a martensite phase of at least about 50 percent by volume and an austenite phase of at least about 10 percent by volume.   
     
     
       2. The process defined in claim 1 comprising the following additional step: c. ageing the material produced in step (b) at a temperature in the range of about 350° C to about 450° C.   
     
     
       3. The process defined in claim 2 wherein, in step (a), the strain is about 10 to about 80 percent and, in step (b), the strain is about 10 to about 60 percent, the temperature is less than about minus 100° C, and the product of step (b) has a martensite phase of at least about 60 percent by volume and an austenite phase of at least about 10 percent by volume. 
     
     
       4. The process defined in claim 3 wherein step (c) is carried out at a temperature in the range of about 375° C to about 425° C. 
     
     
       5. A crystalline microstructure consisting essentially of a stainless steel alloy of the AISI 300 series having a martensite phase of at least about 50 percent by volume and an austenite phase of at least about 10 percent by volume and wherein the tensile strength of the alloy is at least about 190,000 pounds per square inch where the microstructure contains 50 percent martensite by volume and the tensile strength increases by at least about 2000 pounds per square inch for each additional percent of martensite above 50 percent. 
     
     
       6. The microstructure defined in claim 5 wherein the martensite phase is at least about 60 percent by volume and the austenite phase is at least about 10 percent by volume. 
     
     
       7. A method for improving the strength-toughness characteristics of wire or strip, the composition of which consists essentially of a stainless steel alloy of the AISI 300 series, having an Md temperature of no higher than about 100° C and an Ms temperature of no higher than about minus 100° C comprising the following steps: a. deforming the wire or strip at a strain of at least about 10 percent and at a temperature in the range of about Md minus 50° C to about Md plus 50° C, said Md temperature being that of the alloy undergoing deformation, in such a manner that the wire or strip has a martensite phase of no greater than about 10 percent by volume and an austenite phase of at least about 90 percent by volume; and   b. stretching the wire or strip uniaxially at a strain of at least about 10 percent and at a temperature no higher than about minus 75° C in such a manner that the wire or strip has a martensite phase of at least about 50 percent by volume and an austenite phase of at least about 10 percent by volume.   
     
     
       8. The process defined in claim 7 comprising the following additional step: c. ageing the material produced in step (b) at a temperature in the range of about 350° C to about 450° C.   
     
     
       9. The process defined in claim 8 wherein, in step (a), the strain is about 10 to about 80 percent and, in step (b), the strain is about 10 to about 60 percent, the temperature is less than minus 100° C, and the product of step (b) has a martensite phase of at least about 60 percent by volume and an austenite phase of at least about 10 percent by volume. 
     
     
       10. The process defined in claim 9 wherein step (c) is carried out at a temperature in the range of about 375° C to about 425° C. 
     
     
       11. The process defined in claim 1 wherein the material is a stainless steel alloy of the AISI 300 series. 
     
     
       12. The process defined in claim 2 wherein the material is a stainless steel alloy of the AISI 300 series. 
     
     
       13. The process defined in claim 3 wherein the material is a stainless steel alloy of the AISI 300 series. 
     
     
       14. The process defined in claim 4 wherein the material is a stainless steel alloy of the AISI 300 series.